1. Field of the Invention
The present invention relates to a magnetic sensor using a magnetoresistive effect element including a pinned layer and a free layer, a production process of the magnetic sensor and a magnet array suitable for the production process.
2. Description of the Related Arts
Conventionally, a magnetoresistive effect element is applied to a magnetic sensor. For instance, the magnetoresistive effect element includes a giant magnetoresistive effect element (GMR element) or the like that is provided with a pinned layer having magnetization pinned (fixed) in a predetermined direction and a free layer in which the direction of magnetization is changed according to an external magnetic field and that presents a resistance value according to a relative relationship between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer. In the magnetic sensor of this type, it is required that the direction of magnetization in each magnetic domain in the free layer in case where the external magnetic field is not applied to the magnetic sensor is stably maintained in a predetermined direction (this predetermined direction is referred to as “initial-state direction hereinafter) in order to accurately detect a minute external magnetic field.
In general, a thin free layer is shaped into a rectangle as viewed in a plane and the long side (long axis) of the rectangular is matched to the initial-state direction, whereby the direction in each domain in the free layer is matched to the initial-state direction by utilizing a shape anisotropy in which the direction of magnetization is aligned in the longitudinal direction. Further, a bias magnet film that is a permanent magnet film is disposed at both end sections of the longitudinal direction of the free layer to apply the magnetization in the initial-state direction to the free layer so that the direction of magnetization in each magnetic domain in the free layer is returned to the initial-state direction, with a long-term stability, whenever the external magnetic field disappears (see. Japanese Laid open publication 2002-299728 (FIGS. 42-44)).
The state of magnetization in the free layer and the bias magnet film as described above will be explained with reference to FIG. 17 that is a plan view of the free layer and the bias magnet film. In FIG. 17, a free layer 100 is formed to have a longitudinal direction in an X-axis direction, and a pair of bias magnet films 101 and 102 are arranged at both ends of the longitudinal direction.
At a stage of forming these films, the directions of magnetization in each magnetic domain of the free layer 100 and the bias magnet films 101 and 102 are not aligned to the initial-state direction that is the longitudinal direction of the free layer as shown by arrows in FIG. 17A. When an external magnetic field whose magnitude is changed in a direction (Y-axis direction) perpendicular to the longitudinal direction of the free layer is applied to the magnetic sensor in which the free layer 100 and the bias magnet films 101 and 102 are in above-mentioned state, for measuring a resistance value of the magnetic sensor, a hysteresis occurs as shown in FIG. 18A. As apparent from this, in the magnetic sensor wherein the directions of magnetization in the free layer 100 and the bias magnet films 101 and 102 are not aligned to the longitudinal direction of the free layer, the resistance value for the external magnetic field being in the vicinity of “0” fluctuates in a range shown by an arrow in FIG. 18A, resulting in that the magnetic sensor cannot accurately detect a minute magnetic field.
Subsequently, when a magnetic field having a magnitude greater than a coersive force Hc of the bias magnet films 101 and 102 is applied in the longitudinal direction (X-axis positive direction) to the magnetic sensor in which the free layer 100 and the bias magnet films 101 and 102 are in a state shown in FIG. 17A in order to perform an initialization of the free layer 100 and the magnetization of the bias magnet films 101 and 102, the directions of magnetization in each magnetic domain in the free layer 100 and the bias magnet films 101 and 102 are matched to the initial-state direction as shown in FIG. 17B.
When an external magnetic field whose magnitude is changed within a range smaller than the coersive force Hc of the bias magnet films 101 and 102 in the Y-axis direction is applied to the magnetic sensor which is in the above-mentioned state, the direction of magnetization in the magnetic domain in the free layer 100 is changed as shown in FIG. 17C, and then, after eliminating the external magnetic field, the direction of magnetization in each magnetic domain in the free layer 100 is returned to the initial-state direction as shown in FIG. 17D like that as shown in FIG. 17B. When the resistance value of the magnetic sensor is measured in this case, the hysteresis is decreased, so that the resistance value for the external magnetic field being in the vicinity of “0” becomes approximately constant. Accordingly, the magnetic sensor having the free layer 100 initialized and the bias magnet films 101 and 102 magnetized can accurately detect a minute magnetic field.
However, when an external magnetic field having a magnitude smaller than the coersive force of the magnet films 101 and 102 but relatively great and having a main component in the direction (X-axis negative direction) reverse to the initial-state direction is applied to the magnetic sensor (the magnetic sensor having the free layer 100 initialized and the bias magnet films 101 and 102 magnetized), the direction of magnetization in each magnetic domain in the free layer is changed from the state shown in FIG. 19A to the state shown in FIG. 19B, and even if the external magnetic field is eliminated, the direction of magnetization in each magnetic domain in the free layer 100 does not match (return) to the initial-state direction. As a result, the magnetic sensor has a hysteresis again with respect to the external magnetic field, entailing a problem of deteriorating the detection accuracy of the magnetic field.